Process for the manufacture of a synthetic leather



Nov. 4, 1958 J. c. RICHARDS 2,853,570

PROCESS FOR THE MANUFACTURE OF A SYNTHETIC LEATHER Filed Jan. 20, 1956 I ASSOCI-ATING LIQUID SWELLABLE FIBERS WITH POLYMERIC'THERMOPLASTIC B'INDER TO FORM AN INITIAL SHEET HOT PRESSING THE INITIAL SHEET,

SOA'KING THE SHEET IN A LIQUID AT A TEMPERATURE WHICH SOFTENS THE BINDER AND IS ABSORBED BY THE FIBERS CAUSING THE FIBERS TO SWELL COOLING THE SHEET TO SET THE BINDER REMOVING THE LIQUID FROM THE AFIBERSTO CAUSE THEM TO SHRINK INVENTOR JOHN C. RICHARDS AGENT United States Patent PROCESS FOR TIE MANUFACTURE OF A SYNTHETIC LEATHER john C. Richards, Williamsville, N. Y., assignor-to E. I. du Pont tle Nemours and Company, Wilmington, Del., a corporation of Delaware 7 Application January 20, 1956, Serial No. 560,283

1 Claim. (Cl. 18-475) .This invention relates to. non-woven porous fabrics and, more particularly, to a process of preparing and treating a non-woven polymeric fiber sheet to form a synthetic leather.

This application is a continuation-in-part of my copending application S. N. 325,689, now abandoned, filed December 12, 1952. k

In general, the use of a synthetic leather composition in boots, shoes, gloves, etc., is mainly dependent upon its ability to breathe, usually expressed in terms of water vapor permeability or leather permeability (LPV). Physical tests on'the water vapor permeability of leather indicate that leather transpires water vapor about twothirds as readily as free air. In general terms, shoe upper leather samples having a thickness of 0.016- 0.104" have a leather permeability within the range of 2,000-18,000 grams/100 sq. meters/hour when tested according to the method of Kanagy and Vickers Journal of American Leather Chemists Association, 45, 211-242 (April 1950), in an atmosphere of 23 C. and 90% relative humidity.

Hereinafter, the ability of synthetic leather compositions to breathe, i. e., to transpire water vapor, will be expressed in terms of leather permeability (LPV) in grams/100 sq. meters/hour. Based upon comfort tests, the minimum tolerable leather permeability for shoe upper leather is about 2,000 grarns/ 100 sq. meters/ hour. Preferably, for shoe upper leather, the permeability value should be 4,000-20,000 when tested at 23 C. and not greater than 90% relative humidity.

An object of the present invention is to provide a synthetic leather having outstanding breathing qualities. A further object of the presentinvention is to provide a process of preparing a synthetic. leather having outstanding breathing qualities. A further object of the present invention is to provide a process of preparing a synthetic leather having the requisite properties for fabricating boots, shoes, gloves, garments, chair coverings, and other articles wherein a composition capable of breathing is required. A still further object is to provide a process of preparing a synthetic leather having a tenacity, flex life, elongation, tear strength, modulus and leather permeability equal or superior to the various types of genuine leather. Other objects will be apparent from the description given hereinafter.

The above objects are realized by the present invention which, briefly stated, comprises forming a compact, essentially impermeable composite sheet by hotpressing a composition comprising a liquid swellable structural fiber component bound together with a thermoplastic polymeric binder component having a softening temperature below that of the structural fiber component, soaking the composite sheet in a liquid which swells the structural fibers, the liquid being maintained at a temperature between the softening temperature of the polymeric binder and a temperature below the softening temperature of the structural fibers, cooling the composite sheet to set the binder While the structural ice fibers remain in swelled condition, and thereafter finally drying the sheet to remove liquid therefrom and permit shrinkage of the fibers to substantially original dimensons.

The present invention resides in the discovery that swelling the fiber component of a substantially impermeable compacted sheetscomposed of a mat of nonwoven fibers thoroughly impregnated witha thermoplastic polymer, swelling being effected at or above the softening temperature of the polymeric binder, results essentially in stretching the softened binder polymer. That is, as the polymer-encased fibers are swelled, the softened binder polymer around each fiber is deformed in. the direction of swelling; and' thereafter the composite sheet is subjected to a temperature substantially below the softening point of the polymer, this serving to set the binder polymer in deformed condition. This step is followed 'by removal of liquid from the swelled fibers which results inshrinkage of the fibers to substantially the original dimensions thereof. The net result of the process steps of the present invention is believed to be the production of a porous structure in which the pores are interconnecting channels around each structural fiber. The actual volume of voids, based upon the total volume of the sheet, depends-mainly upon the total volume 'of fibers within the sheet and the degree to which the fibers swell. On the other hand, the

magnitude of the internal surface area generated depends primarily upon the number of fibers present in the initial sheet, it being understood that the internal capillaries formed by following the present process are tion of the binder before the fibers are permitted to shrink back to their original dimensions by removal of the swelling medium.

The expression substantially water vapor-impermeable as applied to the initial composite sheet (i. e., before swelling) means that the sheet has a leather permeability value (LPV) of less than 1,000 grams/ 100 sq. meters/hour, measured at 23 C. and relative humidity. In general terms which may be applied to any combination of non-woven fibers and thermoplastic polymers useful in the present process, the water vapor permeability of the initial composite base sheet is substantially no greater than that of homogeneous sheet of the fiber or hinder polymer, whichever has the higher water vapor permeability. Usually, providing the initial composite base sheet is prepared in such a manner that there are substantially no voids present in the sheet, the water vapor permeability'of such a base sheet is intermediate between the permeabilities of homogeneous sheets of the two major components of the base sheet. Actually, the leather permeability values (LPV) of the initial compacted, substantially water vapor-impermeable sheets which are transformed into permeable sheets by following the present process are, low not only because the sheet is compacted by hotpressing, but also because of the thickness of the sheets which are useful for converting to synthetic leather sheets. That is, the initial sheets are usually between 0.0l5"+0.05" in thickness; and homogeneous sheets of either the binder polymer or the structural fiber having thicknesses Within'this range also have low LPVs. Nor- Patented Nov. 4, 1958 3 mally; the LPV is appreciably less than 1,000 and in no case greater.

The term liquid-swellable fiber as employed herein denotes fibers which swell in contact with a liquid by absorption of the liquid into the fibers and which, upon removal of the liquid by drying, shrink back to substantially their original dimensions. The liquid used should not dissolve the binder material although it may be absorbed to a limited extent by the binder.

The softening temperature of the thermoplastic binder polymer is the second order transition temperature (T in the case of a normally amorphous polymer; or, in the case of a crystalline polymer, the softening temperature is the range or thetemperature at which the crystalline phase begins'to disappear as determined by X-ray examination-or any other suitable technique. The second order transitiontemperature of an amorphous polymer is defined" as the temperature at which a discontinuity occu'r s in the curve'bf a firstderivative thermodynamic quantity-of'the polymer with temperature.

In the case of a crystalline polymer, the softening point thereof, as related tothe'temperature at which the swelling step of the present process is carried, out, is the temperature at the lower end of the melting point range. This temperature is further defined as the lowest temperature at which the crystalline structure begins to disappear at an appreciable rate as measured by any of the known techniques, suchas X-ray examination, infra-red studies, or by the measure of heat content as carried out Raine, Richards, and Ryder and reported in Trans. Faraday Soc., 41, 61 (1945). In this study, for example, a sample of polyethylene was found to have a melting point range from about 90 C.-104 C.; i. e., at 90 C. half of the crystalline structure remained.

The single figure of the drawing is a flow sheet i11ustrating the process of this invention.

The following specific examples are given by way of illustration andnot limitation:

EXAMPLEl A leather replacement composition was prepared by carding nylon (polyhexamethylene adipamide) fibers, 1% inch in length and 3 denier per filament, into a loosely bound mat. laural as a binder for the fibers was superposed over the mat of fibers and the assembly was pressed at' a temperature of 160 C. under a pressure of 500 p. s. i. The weight ratio of-binder to fiber was 1/1. The pressing-causedthe binder. to flow, permeate the fibrous mat, encase the fibers and compact the structure. At least some of. the fibers were exposed to the surface. The compacted sheet material'was impermeable to water vapor at this. stage. The compactedsheet material was immersed in water at 95- C. for 60 minutes. fibers absorbedthe hot water, became swollen and deformed the softened binder in the direction of the swelling ofthe fibers. The product was removed from the hot.water, cooled to room temperature and the binder becameharder andset while the fibers were in a swollen condition. Thereafter, the sheet was finally dried to remove the water from the swollen fibers and shrink the fibersto their original unswollen condition. During theldrying. of the fibers the binder remains in its set condition and the fibers shrinkand pull away from thebinderthus forming. interconnecting pores throughout the sheet around and alongsidethe dry fibers.

Thefinished product had the following properties:

Total weight (oz/sq. yd.) 19 Thickness (mils) 40 LPV (grams/100 sq. meters/hour) 3,000 Tenacity (p. s. i.) 3,000 Tensile modulus (p. s. i.) u 6,200- Elongation at. break (percent) Q 103 Tongue-tear strength (lbs.) 14

beginning of the melting point A preformed film of polyvinyl The Tongue-tear strength is measured by cutting a 1" slit in the sheet to be tested, and thereafter measuring the average force in pounds required to propagate the tear. These tests are carried out on standard textile equipment, as described in Federal Specification CCC-T-19lb, dated May 15, 1951.

EXAMPLE 2 A leather replacement product was prepared in the same manner as described in Example 1 by carding a mat of fibers of nylon (polyhexamethylene adipamide), 1%" in length and 3 denier/filament. The mat was impregnated with a copolymer of ethylene/vinyl acetate having a mol ratio of 2.4/1 by placing a preformed film of the copolymer over the mat and pressing the assembly at 120 C. and under a pressure of 500 p. s. i. The weight ratio of fiber to hinder was l/l. The copolymer was caused to fiow and permeate the mat to form a compacted sheet. The compacted film was immersed in water at C. for 60 minutes to swell the fibers and deform the softened binder in the direction of the swelling of the fibers. Thereafter the sheet was cooled to set the binder while the fibers were swollen. The sheet was finally. dried to remove the water from the fibers and cause them to shrink away from the binder and form pores around and/or alongside the fibers. The finished product had the following properties:

Total weight .(oz./sq. yd.) 22 Thickness. (mils) 51 LPV (grams/ sq. meters/hour) 9,000 Tenacity (p. s. i.) 1,500 Tensile modulus (p. s. i.) 1,400 Elongation at break (percent) Tongue-tearstrength (lbs) 26 EXAMPLE 3 A leather replacement product was prepared in. the same manner described above in Examples 1; and2 using viscose rayon lapfibers 1 /2" in length andan'equal quantity by weight of an ethylene/vinyl acetate copolymer having a mol ratio of 2.4 to 1. A carded mat-of the nonwoven viscose rayon fibers and a superposed film of the copolymer were pressed at a temperature within the range of 100-1 10 C. for 4 minutes. The resulting compacted sheet was placed in water at 60-65 C. for 45 minutes, to cause the fibers to swell, and the binder to soften and deform in the direction of swelling. The sheet was removed from the water and the binder allowed'to harden;and set upon cooling while the fibers were swollen. Thereafter the water was evaporated from the fibers at room temperature, whereby the fibers' shrunk and pulled away from the binder. Apparently, there was aslight orientation of the fiber in one direction as the result of "the carding operation; and this is indicated by thephysical properties as measured in the machine direction (MD) and in .a perpendicular direction thereto (PD).

The finished product had the following properties:

Total weight (oz/sq. yd.) 16 Thickness (mils) 40v LPV (grams/100 sq. meters/hour) 9,000 Tenacity (p. s. L):

MD n 1,100.

PD 1,600 Tensile modulus (p. s. i.)

22.000 Elongation at break (percent) MD, "-"r-.-?'--'.-'--.-

PD 50 Tongue-tear strength (lbs.) 16

The following table contains a tabulation of various textile fibers andbinder polymers which have been composited and treated in accordance with the present invention to form permeable synthetic leather compositions.

In general, the initial compacted sheets were prepared by carding the fibers into a non-woven mat, and superposing a homogeneous film of the binder polymer on the mat and subjecting the assembly to a temperature of about 140 C. and a pressure of 500 p. s. i. The binder permeated the non-woven mat. The leather permeabilities of each of the initial sheets were below 700-1,000 grams/100 sq. meters/hour, and were relatively close to those of a homogeneous film of the binderpolymer. The initial composite sheets were immersed in the indicated swelling liquid for a period ranging from 30-45 minutes at the temperature specified. Immediately following swelling of the encased fibers at or somewhat above the softening temperature of the binder polymer, but below the softening temperature of the fibers, the compacted sheet, was cooled to room temperature or at a temperature substantially below the softening temperature of the binder polymer, usually at least 30-40 C. below the softening temperature to set the binder while the fibers are swollen. Thereafter the sheet was dried until substantially all of the absorbed liquid was removed, whereby the fibers were shrunk and pulled away from the binder to form interconnecting pores.

Any fiber capable of being swollen by a liquid and shrinkable to substantially its original dimensions on removal of the liquid, and having a deformation temperature higher than the softening temperature of the binder employed, is useful as a structural fiber in the non-woven porous compositions of the present invention. The particular choice of fiber depends mainly upon the end use of the product and the particular binder being employed. Fibers of nylon, i. e., synthetic linear polyamides such as polyhexamethylene adipamide, polyhexarnethylene sebacamide, polycaproamide and interpolymers thereof, etc., are outstanding for use in the present compositions. Various other types of polyamides and synthetic linear condensation polymers'which may be employed in the form of fibers in this invention are described in U. S. Patents 2,071,250, 2,071,251, 2,071,253, 2,130,948, 2,224,037 and 2,572,844. In general, nylon'structural fibers produce permeable compositions having high tear strength, tensile strength and flex life. Other fibers useful for purposes of this invention include polyethylene terephthalate, viscose rayon, cellulose acetate, polyacrylonitrile; copolymers of acrylonitrile and another ethylenically unsaturated monomer such as, e. g., vinyl chloride Textile LPV Fiber Thick- (Grams/ Example Textile Fiber Denier Binder Treatment ness Oz./sq. 100 sq. (Demer/ (Mils) yd. meters/ Filahr. ment) Nylon Polyethylene 95 0. in water 14 9 7, 615 d ,(1 None 9 18 Viscose Rayon 95 C. in water 19 7 650 uni C. in water..- 15 7 690 50 Viscose Rayon/50 Nylon. 95 C. in water 28 12 7, 650 Polyethylene terephthalate. do 14 10 750 do 27 14 4, e50 25 C. in water. 22 14 260 d0 None 22 14 620 Plasticized Vinyl 85 0.111 water 26 20 6.

Chloride/Vinyl Acetate Copolymer.

I Polyhexamethylene adipamide.

In all cases (Examples 4-13) equal parts by weight of fiber and binder were used. In comparing Examples 4 and 5, the improvement in leather permeability imparted by following the process of the present invention is obvious. Furthermore, Example 5 illustrates the substantially impermeable nature of the initial compacted sheet. The improvement in leather permeability is also illustrated by comparing Examples 6 and 7, and comparing Examples 10, 11 and 12. It is also clearly indicated that swelling of the fibers must be carried out at or above the softening temperature of the binder polymer. Generally, it is not necessary to carry out swelling at temperatures appreciably above the softening temperature of the binder polymer; and the lowest temperature at which binder deformation may be effected is generally employed.

The leather permeability of each of the resulting sheets was measured in accordance with the method of Kanagy and Vickers, Journal of American Leather Chemists Association, 45, 211-242 (April 1950) in an atmosphere of 23 "v C. and 80% relative humidity.

A comparison of the permeable sheets illustrated in the foregoing examples with typical average samples of genuine leather for shoe uppers is presented in the fol lowing table:

1 Includes horse, cow, kip and. middle split.

and vinylidene chloride; cotton and wool, and mixtures of two or more of such fibers.

Since the structural fibers are serving to reinforce the binder material and provide for the formation of interconnecting capillaries or pores having substantially the same interconnecting network pattern as that of the structural fibers, selection of fibers of a particular length and denier must be considered with these two functions in mind. For the purpose of providing adequate tensile strength, tear strength and flex life, the structural fibers should be at least about 0.5" in length. As a general observation, the strength properties of the resulting compositions do not increase appreciably when using structural fibers greater than about 1.5" in length. On the other hand, from the standpoint of handling non-woven mats of fibers on standard textile machines, it may be more convenient to use longer fibers, for example, as long as 8". With respect to the network of interconnecting pores formed throughout the cross-section of a :sheet treated in accordance with this invention, the use of very short fibers, e. g., 0.01" flock, does not produce .a composition having optimum permeability. When the length of the structural fiber is increased from 0.25 to 9.5", the permeability appears to increase; but no appreciable increase is obtained when fibers longer than 0.5" are employed.

It is within the scope of the present invention to employ fibers having a denier within a relatively wide range. Normally, textile fibers having a denier within the range from 1-3 denier/filament are employed. On theother {,hand, fibers having a denier as low as 5.5 10* denier/ dilament have been used in conjunction with fibers of ,greater denier, e. g., 1-3 denier/filament. Usually, it is 7 entirely practical to employ very low denier fibers so long =as they-cohesive bonds within the fibers aregreater than stantially greater than about.16 .denier/filament,.,these fibersbeing relatively stiff and bristle-like.

It is to be understood that the non-woven fibrous mats employed in preparing the polymer-impregnated, substantially impermeable, initial sheets may be fabricated in accordance with any well known batch-wise or continuous techniques such as by carding machines, air deposition apparatus and water deposition or paper-making. tech- .niques.

Furthermore, the resulting fibrous mats may have their component fibers oriented substantially in one direction or randomly arranged. Individual mats having the fibers oriented in one direction may. be crosswlaminated. In any event, thefibers must beinterconnecting so that the resulting capillaries or void spaces aroundthe fibers will be interconnecting.

The binder polymer must be thermoplasti in the sense that it must flow under the hot-pressing step which compacts the initial sheet comprising non-woven-fibers impregnated with a binder. Furthermore, it must be thermoplastic in the sense that it is capable of being deformed at its softening point by the swelling fibers; and the deformation must be substantially set upon subsequent coolf ing of the sheet. Thermoplastic polymerswhichare partially elastic or elastomeric are useful in the'formation of the synthetic leather composition products of this invention. Those polymers which are only partially elastic or elastomeric and are not strictly classified as elastomers include ethylene polymers such as polyethylene, chlorinated polyethylene, chlorosulfonated polyethylene; vinylidene chloride/acrylonitrile copolymers; various polyamides such as N-methoxymethyl polyhexamethylene adipamide and other similar polyamides described in U. S. P. 2,430,860; copolyesters made from ethylene glycol, terephthalic acid and sebacic acid of the general types described and claimed in U. S. Patents 2,623,031 and 2,623,033; polyvinyl acetals such as polyvinyl butyral and polyvinyl laural; ethylene/vinyl acetate copolymers in which the ratio of ethylene to vinyl acetate ranges-from 1.4:1 to 11:1; vinyl chloride resin such as polyvinyl chloride, and copolymers of vinyl chloride with another ethylenically. unsaturated monomer such as vinyl acetate andvinylidene chloride. All of the above polymers may be employed. with or without plasticizers.

'TheJbinder material may be incorporated into the initial impermeable composite sheet in a variety of Ways. A preferred practice is to press mats of carded fibers and superposed homogeneous films of the binder material. Or, the bindermaterial, too, may be in the form of fibers which may be formed into arnat along with the structural fibers to form a composite sheet by pressing the carded mixed fibers at elevated temperatures. A fibrous mat ready for compacting by.hot-pressing may be formed by mutual coagulation of a mixed dispersion of structural fibers and a binder polymer. Fibrous mats of nonwoven fibers may be impregnated with a binder polymer from an aqueous dispersion of .the binder polymer. Other techniques of incorporating the binder" material include impregnation of a fibrous mat with a.binder polymer in solvent solution, and impregnation of a fibrous matwith a binder polymer in the form of a powder. Non-woven fibrous mats may also be impregnated from hot melts of the binder polymer or by calendering a polymerlinto a fibrous'mat. "Regardless of the technique employed to form a composite sheet, the-sheet 'is'consolidatedprior to treatment in accordance with the process 8 .steps.of.thepresentinvention by pressing at a temper- .ature. which-.isnabovethe.flw..temperature of the binder .and .below the..'sottening: temperature: of the structural fibers.-...The-.pressure used should besuificient tocause the. binder tosflowsand thoroughly impregnate the fibrous components of .thesheet.

v.An important: consideration with respect to choosing- ,a.particular.=-combination of fiber and binder is the softeningtemperatures of the-'fibers and binders. For'pracl0 tical operations ..the.'.binder should soften at' a'temperature aLleast 50 F. lower than the 'softening'temperature' of the structural fibers.. ..The.magnitude of adhesion betweenihe fibenand .binder must. be considered. If;-for example, .the..adhesion between-fibers and binder is con- 15 siderable; the .fibers upon .shrinkage to substantially original. .dimension,.may..tend to 'pullthe binder material back to .itspriginal. position, .therebynot forming voids around the .=shrunken..fibers. Inother words, .the 'fibers' during shrinkageemust break-awayfrom contact with the binder 0 material;.and. strong .adhesivezbonds between fiber and bindenmayprevent this. If the adhesion is too great, the fibersmay-be pulled apart within the structure and be prevented from shrinkinguniformlyback to substantially original dimension. Another consideration with respect to adhesion between fiber and binder is concerned with the routetaken by the liquid in entering the body of the fibers to effect swelling. It is important in the fabrication and compacting of the initial sheet that the fibers be dispersed uniformly throughout the cross-section of the-sheet, and that at least'a substantial portion of the fibers protrude through the'surfaces of the compacted sheet. Upon soaking the sheet in a liquid, theliquid is absorbed into the fibers by capillary action; and the liquid enters the fibers positioned within the internal crosssection of the sheet at places where the fibers from the surface interconnect with the internal fibers. On the other hand, in the case of using fibers and a binder forming low adhesive bonds therewith, the liquid may tend to force its way around openings which may result from poor adhesion between the fiber and binder. In this manner, the liquid may enter the fibers at a more rapid rate. than .merely by-capillary action within the fibers and, hence, reduce the soaking time necessary to swell the fibers.

4.) As a method of evaluating the relative adhesion between various. binder materials andfiber components, sheets of nylon, i. e., *polyhexamethylene adipamidyfilm, have. been fastened together by means of insertinga film of selected binder materials therebetween, and pressing 00 the layers at vsubstantially'thesame temperature and pressureconditions employed to prepare the initial: composite sheets, i. e., before the swelling action'of. the presentv invention. Thereafter, the seals were pulled in a textile tensile .testing machine to obtain the in grams necessary to separate the layers of nylon film. The following presentsthe results of these tests.

Heat seal value adhesion Binder polymer: (grams) Polyisobutylene 39 Polyethylene 0 MCA/VA 1 526 MCA/AN 2 536 E/VA 456 DCD 4 669 Nylon 5 1,416

*Nylon=polyhexamethvlene adipamide. 1 MCA/VA=Methyl Cellosolve acryIate/vinyl acetate copo1ymer.

. iMCAlANzmethyl Cellosolve acrylate/acrylonitrile cop0 ymer.

E/VA: ethylene/vinyl acetate copolymer.

4 DCD=modifled dichlorobutadiene. 7 5 Nylon=-N'-m'ethuxymethvl' polyhexamethylene adipamide.

It is to be understood that the purpose of the adhesion test is to provide a method of selecting or screening various combinations of binders and fibers which exhibit little or no adhesion under the conditions required to composite the components to form the initial sheet. For example, as shown in the above table, polyethylene does not adhere to nylon film under the conditions employed to form a composite sheet of nylon fibers impregnated with polyethylene, e. g., inserting a mat of intertangled nylon fibers between adjacent films of polyethylene and pressing the composition at about 125 C. and under 500 p. s. i. On the other hand, N-methoxymethyl polyhexamethylene adipamide forms a very strong adhesive bond with polyhexamethylene adipamide; and a composite composed of polyhexamethylene adipamide fibers impregnated with N-methoxymethyl polyhexamethylene adipamide may not be rendered permeable by the normal conditions of the present invention. In general, the process of the present invention will not produce satisfactory permeability when employing combinations of fibers and binder material which exhibit adhesion greater than about 750 grams when evaluated in accordance with the test described above.

The main objective in subjecting the initial composite sheets to the process steps of this invention is to bring about a reversible dimensional change in the structural fibers thereby producing a permanent deformation of the binder material in the portions surrounding the structural fibers. Therefore, the fibers employed must become readily swollen in the particular liquid employed, e. g., water, which liquid must not dissolve the fibers or the binder material to any appreciable extent. On the other hand, the liquid may effect some swelling of the binder material so long as the net result is permanent deformation of the binder material to form capillaries or voids around the structural fibers. The preferred liquid in which the initial composite sheets are soaked to swell the structural fibers is water. In instances where the softening temperature of binder material is above 95-100 0., other suitable liquids may be employed so long as they are readily removed from the fibers and surfaces of the treated sheets. For example, various water-soluble dihydric and trihydric alcohols and aqueous solutions thereof may be employed, such as ethylene glycol and various polyethylene glycols and glycerin. On the other hand, in order to employ liquids which are readily removable from the surfaces of the sheets by water and yet maintain a temperature above 100 C., solutions may be prepared by dissolving various salts such as sodium chloride, sodium acetate, etc., in water. Such swelling media are non-toxic and cheap. To maintain water at temperatures above 100 C., swelling may be effected in a pressure tank or vessel. As a general requirement, the liquid employed for soaking the initial sheets should not dissolve the structural fiber or binder material to any appreciable extent; and the swelling produced must be due to the presence of the liquid within the fibers so that shrinkage of the fibers may be effected by removal thereof from within the fibers.

Theoretical analyses, based upon extensive experimentation, relating to the mechanism of the transmission of water vapor through leather and synthetic leather com positions, indicate that water vapor is transmitted not only by gaseous diffusion but also by conduction. Transmission by gaseous diffusion is mainly influenced by the total volume of void space; Whereas, transmission by conduction or by capillary action is influenced by the magnitude of the internal surface area. With respect to the present process of forming a permeable composition, the internal surface area generated will depend chiefly upon the proportion, and particularly the actual number of individual structural fibers in the initial composition. On the other hand, the total volume of void space will depend upon the proportion of fibers and the degree to which the fibers are swelled. Another con- 10 sideration with respect to the proportion of structural fibers in the initial compacted sheets is the strength and flexibility of the resulting permeable sheets. Preferably, from the standpoint of strength, flexibility and perme ability, approximately equal volumes (equal weights where density of fibers and binder are substantially the same) 'of structural fiber and binder are normally employed. Strength properties and leather permeability decrease appreciably when less than 20% structural textile; fibers by volume of the total composite is employed.- On the other hand, compositing more than 70% textile fibers by volume results in the formation of a highly permeable sheet having insignificant strength properties.

The process of the present invention is exceptionally versatile with respect to preparing a synthetic leather composition of the desired internal structure and surface texture. For example, embossed sheets may be readily and eificiently produced by employing embossing rolls or pressure-applying surfaces during compacting of the combination of structural fibers and binder material. Hence, embossing may be carried out in conjunction with a necessary step in the process, or, it may be carried out immediately thereafter under conditions of lower temperature and pressure than employed in the compacting step. The embossed surface imparted by this treatment remains substantially intact after subsequent swelling and shrinking of the structural fibers. Embossing the permeable product results in substantially decreasing the permeability of the structure and is generally to be avoided.

Another outstanding advantage of the present process. is that the cross-sectional structure of the synthetic compositions may be tailored for the desired end use by laminating or fusing different plies of one or more of the: two basic components together. Various plies contain ing one or more of the two basic components, i. e., fibers and binder polymer, may be laminated together to control the face-to-back concentration of structural fiber and binder material. For example, a top ply of the initial sheet may be composed entirely of the binder polymer in the form of a homogeneous film. The middle ply may be composed of various amounts of the structural fibers and binder polymer; and the bottom ply may be composed essentially of structural fibers. Such a structure after lamination and compacting by hot-pressing, followed by the swelling and the shrinking steps of this process, has a relatively smooth or non-fiber-like upper surface (usually referred to as the skin side) and a bottom side which is substantially fibrous (usually referred to as the flesh side). It is obvious that the concentration of the structural fiber and binder material may be varied from the face-to-back of the sheet within reasonable limits by employing the general practices within the scope of the present invention.

While there are above disclosed but a limited number of embodiments of the structure, process and product of the invention herein presented, it is possible to produce still other embodiments without departing from the inventive concept herein disclosed, and it is desired therefore that only such limitations be imposed on the appended claims as are stated therein, or required by the prior art.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as.

follows:

The process of preparing a flexible synthetic leather which comprises the steps, in sequence, of associating liquid-swellable structural fibers at least about 0.5 inch in length and a thermoplastic polymeric binder to forman initial sheet, said thermoplastic binder having a softenhot pressing said initial sheet at a temperature above the softening temperature of the binder and below the softening temperature of the fiber to form a substantially watervapor impermeable sheet, soakingethensheetx-inrat-liquid; at a temperature abovethesoftening temperature-lofthe binder andbelow1thelsoftening.temperature of:the; fiber to swell the fibers eandu deform the fibers'linthe direciton of swelling the fibers, said liquid being absorbable by thelfib'ers andhavingno deleterious effect-on the binder, substantially completely 'settingthe binder .by cooling vwhile the fibers remain in swelled condition, .and, removing the liquid from the swelledfifib'ers after the bindertis substantially completely setto shrinktthel fibers;

awaytfrom the 'bindervtoiformra vapor-permeable :sheet haying an interconnecting network patternof pores essentially thesameasthe interconnectingnetwork patternof fibersw distributed throughout the binder;

' References Cited in the fileof this patent UNITED STATES PATENTS I 2,274,231 Behrman Feb. 24, 1942 

